The invention relates generally to a cordless position detection apparatus or digitizer, and in particular to a digitizer that does not require a battery.
Nowadays position detection apparatuses also known as graphic digitizers are well known in the art to input the coordinates of a point into a computer. In contrast to a common computer mouse in such a position detection apparatus the absolute position of a pointer or pointing instrument (pen or cursor) with respect to a working surface or tablet (position detection area) is converted into electrical signals.
From U.S. Pat. No. 5,045,645 and European patent 0 259 894 a wire- or cordless pen or cursor, not requiring a battery is known. By not using a battery in the pen costs, space and additional weight can be saved on. Furthermore, a discharged battery has not to be recharged or replaced. A cordless pen allows a free and comfortable operation of the pen over and across the table. Applications of such a position detection apparatus are for example in the construction, navigation or computer arts. Recently, applications of the so-called "pen computing" dealing with input into a computer by means of a pen are interesting applications requiring a position detection apparatus. The physical principle underlying a position detection apparatus using a cordless pen is a well-known principle from electro dynamics, namely that electromagnetic signals can be transmitted from one wire or coil to another wire or coil by means of induction.
The amplitude and phase of the transmitted signals depends on the relative position of transmitter and receiver and can hence be used for a localization of the transmitter and a receiver, respectively.
From WO 92/21 082 A1 a pointer (mouse or pen) is known that comprises a circuit including a memory in which data are stored for protection of access to an electronic device, for instance characteristics of the signature of a user entitled to use the electronic device.
From Japanese open-laid publication JP 05-113844 A a cordless and batteryless pen is known, the supply energy of which is transmitted and induced, respectively, from the outside.
European patent 0 357 397 teaches a pen with a plurality of coils that can be distinguished by the wires leading to the coils. Using these coils, a rotation of the pen can be recognized. Also non-circular shaped coils are used but only to save on space. It is not disclosed in this document to use the non-circular shaped coils for detection of a rotation of the pen.
From European patent 0 407 921 A2 a pen comprising a plurality of coils is known by which a rotation of the pen can be recognized. Coils use different frequencies in order to allow a distinction between said coils.
The known cordless and batteryless position detection apparatuses have the following disadvantages:
The position detection apparatus described in European patent application 0 259 894 uses for transmitting position information between the pen and the tablet a transient signal of a decreasing oscillation. The transmitting wire in the position detection area (tablet) has to be switched to a receiving mode after the transmitting signal has been switched off. Hence a quite large amount of time is needed to sample each wire as each transmitting and later receiving wire, respectively, has to excite an oscillation and receives afterwards a signal inductively by the decreasing oscillation of the oscillation circuit in the pen. This reduces significantly the rate to determine positions and to transmit data on positions. Furthermore, the signal of the oscillation circuit decreases exponentially causing inaccuracies in the detection of the coordinates. Furthermore, it is not possible to transmit a plurality of different data being sampled by the pen, i.e. data that do not relate to a position with respect to a tablet, that are to be transmitted to the tablet.
An alternative system is disclosed in U.S. Pat. No. 5,045,645. However, an additional oscillation or resonance circuit is used for the cordless and batteryless transmission of information. In this system also no transmission of a plurality of detected information in the pen is provided, such as thickness of a line, colors or commands for manipulation of drawn objects. The technique for transmission of energy by modulation of a carrier does furthermore generate two side bands, i.e. one below and one above the carrier frequency. This has negative effects on the rate of transmission of coordinates.